The present invention relates to electrical power distribution system. More particularly, the present invention relates to an anomaly detection system using RF technology such as Spread-Spectrum Time-Domain reflectometry (SSTDR) techniques that can identify anomaly (high) impedances that represent faults on electrical distribution circuit and determine where they are occurring.
Incidental high-impedance faults are not currently detected and isolated by conventional means, such as overcurrent relays and fuses, and represent a hazard to unsuspecting bystanders and utility workers. Incidental faults have been shown to occur at a rate of one fault per utility power line every four years. High-impedance faults have a higher occurrence probability in longer distribution circuits.
There are high-impedance fault detection systems that use passive sensing devices to measure the primary voltage, current, and harmonics for power distribution circuits. Some of the available devices can detect a broken utility power line by measuring the current drop on a utility power line. Other available devices are able to detect an arcing event (e.g., a utility power line coming into contact with a tree) through the use of current measurements that match a particular pattern that has been observed. In order to localize the location of a broken and/or arcing conductor, there must be sensing devices on both sides of the event, and the location of the detectable event is only known to be somewhere between the two devices.
A radar (radio detection and ranging) system emits a known radio frequency signal into a medium (typically air) in order to determine the distance to objects of interest. Reflections of the radar signal occur when the medium through which the radar signal is propagating changes (e.g. from air to a solid object). When a radar signal encounters a medium change, some of the energy in the incident radar signal is typically reflected back toward the originating source of the signal. The time between the emission of the signal and the reception of the reflected signal, as well as the direction of the transmitted and reflected energy, can be utilized to determine the position of objects causing the reflections.
Radar imaging combines radar returns focused in spatially unique directions in order to create an image based on the reflections. This image can be referred to as a map.
When radar signals encounter a medium change, not all of the energy is reflected. Some energy is absorbed, some reflects, and some continues to propagate in the original direction. The energy that continues to propagate in the original direction may, in turn, be reflected by yet another medium change, and thus it is possible with radar to “see through” or image through objects.
A Time Domain Reflectometer (TDR) uses principles similar to those employed in radar except that the medium through which the emitted signals travel is a conductor. If a signal is injected into an ideal conductor having a characteristic impedance Z that is terminated at its end by a load having the characteristic impedance Z, there will be no reflected signals. These conditions rarely, if ever, exist under real-world conditions, where impedance discontinuities along the cable-to-load impedance mismatches are common. A TDR system measures signal reflections from impedance discontinuities and the cable-to-load impedance mismatches along a conductor. In order to measure those reflections, the TDR will transmit an incident signal onto the conductor and listen for its reflections. If the conductor is of uniform impedance and does not have terminations or splits, there will be no reflections and the remaining incident signal will be absorbed at the far end by the termination. However, if there are impedance variations, some of the incident signal will be reflected back to the source. A TDR is similar in principle to radar [https://en.wikipedia.org/wiki/Time-domain_reflectometer].
The reflection coefficient (the amount of energy that is reflected by a discontinuity in the transmission medium) is the ratio of energy that returns relative to the incident energy. This is often called the impedance change or impedance mismatch.
Impedance mismatches are defined as metal-to-metal contacts on a conductor that have an impedance value that is different from the original conductor. When a TDR signal that is tuned to the conductor impedance is applied to the contact, a portion of that energy will be reflected back to the source of the signal. As noted in U.S. Patent Publication 20160124449 (Heenan, Abbott, Ragsdale) a reflection in a conductor may be an incidental high-impedance reflection point.
In power engineering, measurement of unexpected impedance values is a classic way to determine faults in a metallic conductor. When impedance measurements return unexpected values that indicate the possibility of a problem, they are referred to as impedance faults.
Spread Spectrum Time Domain Reflectometry (SSTDR) is a measurement technique that has been used to identify faults, usually in electrical wires, by observing reflected spread spectrum signals. This type of time-domain reflectometry can be used in various high-noise and live environments. For accurate location of a fault in a wiring system, the SSTDR associates the pseudo noise (PN) code [or spreading signal] with the signal on the utility power line then stores the exact location of the correlation before the arc dissipates [https://en.wikipedia.org/wiki/Spread-spectrum_time-domain_reflectometry].